<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            display: flex;
            flex-wrap: wrap;
        }

        .box div {
            flex-grow: 1;
            flex-basis: 500px;
        }
    </style>
</head>

<body>
    <div class="box">
        <div style="background: skyblue">1</div>
        <div style="background: pink">2</div>
        <div style="background: paleturquoise"> 3</div>
        <div style="background: peru">4</div>
        <div style="background: peachpuff">5</div>
        <div style="background: plum">6</div>
    </div>
</body>
<script>

</script>

</html>